Decentralized doesn't mean every user needs to run their own node. Getting people to run a node is solved once there's a (financial) incentive for that, which is what made cryptocurrencies possible.

I agree no service is going to get adoption if its main feature is decentralization, but that doesn't need to be its value proposition. "Post your cat photos here and get paid in cat-tokens that are worth real money" would be the incentive.